Structure, function and druggability of the African trypanosome flagellum

Abstract

African trypanosomes are early branching protists that cause human and animal diseases, termed trypanosomiases. They have been under intensive study for more than 100 years and have contributed significantly to our understanding of eukaryotic biology. The combination of conserved and parasite-specific features mean that their flagellum has gained particular attention. Here, we discuss the different structural features of the flagellum and their role in transmission and virulence. We highlight the possibilities of targeting flagellar function to cure trypanosome infections and help in the fight to eliminate trypanosomiases.

Keywords: African trypanosomiasis; chemotherapy; flagellum; model organism; parasite; trypanosome.

Figure 1

Trypanosomes are well‐designed swimmers. Epifluorescence live cell microscopy of (a) an insect stage and (b) a bloodstream stage trypanosome expressing a fluorescently tagged flagellum protein (green) and stained with Hoechst to visualise the DNA (magenta). Note the flagellum is attached to the cell body for most of its length. Trypanosomes swim left to right as shown here. Scale bar = 5 μm. Figure adapted from Dean & Sunter (2020).

Figure 2

The trypanosome flagellum has the canonical microtubule domains. (a) A longitudinal transmission electron microscopy (TEM) section through a detergent‐treated trypanosome cell reveals the three domains of the flagellum delineated by terminal and basal plates. Cross‐sections through the flagellum reveal (b) the basal body triplet microtubules (9 + 0), (c) the transition zone doublets (9 + 0), and (d) the proximal and (e) distal flagellum (9 + 2). Note the PFR that starts approximately 1 µm from the flagellum base. Figure adapted from Vaughan et al. (2006). PFR, paraflagellar rod.

Figure 3

Structures of the transition zone. (a) A longitudinal section through the TZ reveals radial fibres that enter the TZ underlying the TZ membrane (arrowheads) and a filamentous structure in the TZ lumen (arrow). (b) A transverse TEM section reveals the external collarette made up of doublet‐tubules (black arrowheads) connected by fibres (white arrowheads). Fibrous ‘hooks’ penetrate into the lumen from the TZ doublets (arrow). Figure adapted from (Lacomble et al., 2009). TEM, transmission electron microscopy.

Figure 4

The PFR is an extra‐axonemal, structure required for flagellar beat regulation. A cross‐section through the flagellum reveals that the PFR is a paracrystalline structure with a proximal, intermediate, and distal domain, with respect to the cell body. The arrows indicate connections to an axonemal doublet and the flagellar membrane. Figure adapted from Sherwin & Gull (1989). PFR, paraflagellar rod.

Figure 5

The FAZ has a complex cytoskeletal architecture. (a) A cross‐section through the trypanosome cell shows the FAZ domains, including the cell body and flagellar domains. The FAZ filament (arrowhead) links to junctional complexes on the plasma membrane. (b) A cartoon indicating the contrasting assembly sites of the FAZ (proximal) and axoneme (distal) necessitating that the different structures must move antagonistically during flagellum growth. Figure adapted from Sunter & Gull (2016). FAZ, flagellum attachment zone.

Figure 6

The flagella connector allows the old flagellum to template the path of the new flagellum. (a) The cell cycle of insect‐stage trypanosomes. At G1, trypanosomes possess a single kinetoplast and a single nucleus (1K1N) with a single attached flagellum. During the cell cycle, new flagellum elongation coincides with kinetoplast duplication and segregation (2K1N). The new flagellum is physically attached to the old flagellum via the flagella connector. Following mitosis (2K2N) and the initiation of cytokinesis, this connection is released. (b) Detergent treated cells ('cytoskeletons') were negatively stained and mounted whole onto grids for TEM. The flagella connector (asterisk) is positioned at the tip of the new flagellum (nf) and along the side of the axoneme (ax) of the old flagellum (of). Figure adapted from Briggs et al. (2004).

Figure 7

Trypanosomes must traverse the tsetse anatomy to infect new hosts. Upon inoculation, bloodstream form trypanosomes (blue cells) must first colonise the midgut and differentiate to insect procyclic forms (green cells) at the tsetse proventricular lumen (1). They then swim through the peritrophic matrix at the proventriculus to the ectoperitrophic space (2). Some cells become trapped in peritrophic matrix 'cysts' (3) which flow through the tsetse digestive tract (4) and are defecated. After colonising the proventriculus, the parasites swim to the salivary glands where they attach and then reinoculate new hosts. Illustration originally designed by Laura Jeacock and adapted from Rose et al. (2020).

Figure 8

Hydrodynamic flow facilitates surface‐bound antibody clearance. As trypanosomes swim, hydrodynamic forces sweep surface‐bound antibodies to the cell's posterior. Antibody‐VSG complexes are internalised by endocytosis and the VSG is recycled to the cell surface after antibody removal in the endosomal system. Figure redrawn from Dean & Matthews (2007).